1. Field of the Invention
The present invention relates to mixed solvent systems for polyimide materials. The mixed solvent/polyimide systems find particular application in the manufacture of electronic components.
2. Description of the Prior Art
U.S. Pat. No. 4,238,528 Angelo et al discloses a polyimide coating process and a material useful therein where the coating material includes an aromatic polymer which, when cured, forms a polyimide, organic solvents for the polymer including, for example, a combination of 1-methyl-2-pyrrolidone (NMP), NMP/acetone, NMP/Cellosolve, NMP/xylene, NMP/toluene and (2-ethoxyethanol)-Cellosolve/acetone, and a non-ionic fluorocarbon surfactant. Optionally, adhesion promoters such as silanes can be present. There is no disclosure in Angelo et al of using a fluorinated hydrocarbon as per the present invention to achieve a synergistic effect in a system as claimed herein. Since the systems of the present invention can be used with devices as disclosed in this patent, it is hereby incorporated by reference, including the drawings thereof.
U.S. Pat. No. 3,560,426 Adesko et al discloses a coating composition having a low viscosity particularly useful for wire coating which comprises an amine terminated aromatic polymeric precursor of an amide containing polymer and a reactive melamine resin. Various inert organic solvents useful therewith are recited including NMP, N,N-dimethyl formamide, dimethyl sulfoxide, methyl ethyl ketone, etc., which can be used alone or in combination or in combination with poorer solvents such as xylene and toluene. Adesko et al is subject to the same defects as Angelo et al.
U.S. Pat. No. 4,173,683 Comizzoli relates to a method of chemically treating a passivating overcoat of a semi-conductor device immediately prior to encapsulating the device, the treating being with an organosilane.
IBM Technical Disclosure Bulletin, Vol. 20, No. 6, November 1977, page 2401, discloses an improved polyamic acid formulation of superior stability which is formed by mixing a solution of dimethyl sulfoxide containing pyromellitic dianhydride with a solution of 1-methyl-2-pyrrolidone containing diaminodiphenyl ether.
In the manufacture of integrated circuit modules and similar devices it is customary to seal the metal circuitry and connections between a substrate and integrated circuit chip devices with a polymer layer. Typically, a liquid conformal coating is coated over the devices and then cured to protect joints and pad areas against corrosion, migration, atmospheric contamination and moisture. The coating mechanically enhances chip joint reliability while serving as a dielectric insulator and penetrates under those chips having design configurations that contain internal joints/pad that, like perimeter joints/pads, also require coating coverage. The conformal coating can be applied by spin, spray, dip or dispense techniques. The method of application is optional, dependent upon product requirements.
Polyimide coatings have been used in the past as conformal coatings for electronic components and circuitry. However, prior to the present invention problems have been encounted in obtaining uniform coatings of such polyimide materials (due to the poor surface tension effects encountered) and with flammability problems due to the organic solvents used to apply the polyimide precursors which, after drying and curing, yield the objective polyimide coating.
Accordingly, one major objective of the present invention is to significantly reduce the surface tension of conformal protective coatings which greatly facilitates the ability of the coating to uniformly wet all joints and pad areas when, for example, coating modules where integrated circuit chips are joined to the substrate as described in U.S. Pat. No. 3,429,040, hereby incorporated by reference.